Are you going for 1RMs? If that's the case, google "1RM calculators." You'll get a bunch of sites that have one of these; you simply type in the weight you're lifting and the number of reps you're doing with that weight and it will calculate your 1RM.

Of course, your real 1RM will likely be higher because max weight you normally lift is done after a number of sets, so you lift less than you would if you were fresh and rested. However, the value you get is a good starting place.

How many attempts do you get at your 1RM? If you get a few, then just start at the calculated value and move up in weight according to how you feel, just like in a PL contest.

to determine my 1RM I start at my workout weight and progressivly add 20 lbs/set with 3 rep sets. When that becomes taxing add 10lbs per set for 2 rep sets until you can only perform 1 rep. As you approach max make sure you get significant rest between sets (2-3 minutes). It works well for me; a very large part of it is the belief that one is able to move the weight they are about to press. If you have doubt in your mind you will most likely fail when the going gets tough.